Lanthanide Contraction Associated Coordination Regimes and Photocatalytic Performance of Polyoxometalate-Based Lanthanide Phosphonates

Abstract

Lanthanide contraction provides a continuous scale for tuning coordination structures, yet its systematic use in photoactive materials remains underexplored. Herein, 14 POM-phosphonate-lanthanide hybrids from La to Lu (excluding Pm) were synthesized under mild conditions. Single-crystal XRD identifies three coordination regimes: La³⁺ at the eight/nine-coordination boundary, Ce³⁺-Yb³⁺ adopting stable eight-coordination, and Lu³⁺ undergoing a ligand-deficient transition due to steric congestion. The Sm³⁺ complex 5b, obtained under diffusion conditions, exhibits the same motif as Lu³⁺ complex, indicating that crystal growth conditions can influence coordination-mode selection. Combined with band-structure analysis, EIS, radical trapping, EPR, and HPLC-MS, the photocatalytic mechanism was investigated. This work converts lanthanide contraction from a descriptive concept into an experimental framework for constructing structure-performance correlations.